Electrostatic speakers generate sound pressure when a driving signal is input to a fixed electrode and a DC bias voltage is applied to a diaphragm. The diaphragm is charged with a higher voltage than the fixed electrode, and when an audio signal is transmitted to the fixed electrode, an electrostatic field is generated in response to the audio signal. The variable electrostatic field generated around the fixed electrode interacts with a fixed electrostatic field of the diaphragm which causes the diaphragm to be both pulled and pushed, resulting in the generation of sound.
The electrostatic speakers include a single ended speaker having a diaphragm and a single fixed electrode that are arranged in parallel to each other, and a double ended speaker having a diaphragm and a pair of fixed electrodes arranged at both sides of the diaphragm.
FIGS. 1 and 2 show the schematic structure of a general single ended speaker. The speaker includes a sound generator 10 having a diaphragm 12 and a fixed electrode 14, and a plurality of holes 14a are formed in the fixed electrode 14.
In addition, insulating supports 18 are formed between both ends of the diaphragm 12 and the fixed electrode 14. The level of an audio signal input from an amplifier (not shown) is increased by an audio frequency transformer TF.
One output node of the transformer TF is connected to the fixed electrode 14, and the other output node thereof is connected to a negative pole node of a bias voltage source VB. An positive pole node of the bias voltage source VB is connected to the diaphragm 12. The bias voltage source VB makes the speaker charged and maintained at a consistent charging capacity.
The diaphragm 12 has a consistent voltage and a consistent charging capacity in an equilibrium state, and a voltage V is applied between the diaphragm 12 and the fixed electrode 14. The voltage between the fixed electrode 14 and the diaphragm 12 varies according to an amplified audio signal, and then the balance between attractive force and repulsive force applied to the diaphragm 12 is upset, which causes the diaphragm 12 to move, resulting in the generation of sound.
FIG. 3 shows the schematic structure of a general double ended speaker. The double ended speaker is formed by adding one fixed electrode to the sound generator of the single ended speaker, in order to improve the driving force of the diaphragm. That is, the double ended speaker includes a sound generator 20 having two fixed electrodes 24 and 26. In this way, the double ended speaker reduces the distortion of sound and increases the level of sound pressure.
In the double ended speaker, the bias voltage VB is applied between a diaphragm 22 and an intermediate node of a secondary coil of the transformer TF. When amplified audio signals are input to two fixed electrodes 24 and 26, the diaphragm 22 receives the attractive force from one of the two fixed electrodes 24 and 26 and the repulsive force from the other fixed electrode to vibrate, thereby generating sound.
A plurality of holes 24a and 26a are formed in the fixed electrodes 24 and 26, respectively, and insulating support members 28 are formed between the diaphragm 22 and the fixed electrodes 24 and 26.
In addition, the diaphragm used for the electrostatic speaker should have high surface resistance such that positive charges that are charged with a high voltage can be stably maintained in the diaphragm, and should appropriately prevent the discharge between the diaphragm and the electrodes caused by a high voltage applied to the diaphragm. In addition, the diaphragm should have a strong initial tensile strength in order to prevent the diaphragm from being attracted to the electrode when no signal is input and the positive charge on the diaphragm from being discharged.
In order to satisfy these characteristics, the diaphragm for an electrostatic speaker according to the related art is formed as follows: a petrochemical polymer film, such as a thin polyester film having a relatively strong tensile strength, is stretched to be adhered to a frame; a heterogeneous material, such as nylon or PVC, is coated on one surface of the polyester film in order to reduce low-frequency resonation within an audible frequency range; and high-resistance conduction coating and antistatic coating using a diluted solution of graphite powder is performed on the other surface of the polyester film.
However, the diaphragm obtained in this way is non-ventilative, and has the following problems. That is, when the intensity of a signal input to the fixed electrode increases and the diaphragm is close to the fixed electrode, a ground effect is generated.
The ground effect means that, when an object rapidly approaches the ground, high air resistance occurs between the ground and the object within a specific range of distance from the ground, which prevents the object from approaching the ground.
For example, flying objects, such as a helicopter and a GEV (ground effect vehicle), use the characteristics of the ground effect. When a helicopter hovers near the ground, the generated air hits the ground, causing the air between the helicopter and the ground to be compressed. The compressed air serves as a cushion that helps the helicopter to hover.
This effect is called the ground effect, which saves the power of the helicopter. The height incurred by the ground effect corresponds to the diameter of a main rotor while the helicopter is hovering.
FIG. 4 is a graph illustrating the relationship between the ratio of the height of the rotor to the diameter of the rotor (horizontal axis) and the ratio of an increment in the thrust of the rotor (vertical axis). As shown in FIG. 4, as the height of the rotor decreases, the helicopter is more affected by the ground effect, and the increment in the thrust of the rotor is about 20% when the ratio of the height of the rotor to the diameter of the rotor is 0.25. In addition, the increment in the thrust of the rotor drastically decreases to about 7% when the ratio of the height of the rotor to the diameter of the rotor is 0.5. Further, the increment in the thrust of the rotor is 0 when the ratio of the height of the rotor to the diameter of the rotor is 1.25, and the helicopter is not affected by the ground effect.
The principle of the ground effect can be applied to the electrostatic speaker, which will be described below. That is, when a distance d1 between the holes formed in the fixed electrode corresponds to the diameter of the rotor of the helicopter and a distance d2 between the fixed electrode and the diaphragm corresponds to the height of the rotor of the helicopter, a variation in the thrust of the helicopter according to the relationship between the diameter of the rotor and the height of the rotor corresponds to a pressure variation between the diaphragm and the fixed electrode according to the relationship between the distances d1 and d2.
Therefore, as the diaphragm becomes close to the fixed electrode, it is more difficult for the air between the diaphragm and the fixed electrode to pass through the holes formed in the fixed electrode. As a result, the diaphragm has high air resistance, and sound pressure is lowered.
The reduction in the sound pressure becomes larger as the intensity of an input signal increases and the diaphragm becomes closer to the fixed electrode. In addition, as an input frequency is lowered, the amplitude of the diaphragm increases when a signal having the same intensity is input. In this case, the diaphragm is less affected by the ground effect. Therefore, the intensity of the output sound becomes lower as the frequency decreases, which makes it difficult to obtain a uniform frequency reproduction characteristic.
In order to overcome the ground effect, a method of increasing the ratio of the holes to the fixed electrode (the number of holes per unit area) or a method of increasing the distance between the diaphragm and the fixed electrode may be considered. In the former case, an area in which electrostatic attractive force is generated is reduced, and the electrostatic attractive force is weakened, which results in a reduction in reproduction sound pressure. In the latter case, when the distance between the diaphragm and the fixed electrode increases, the electrostatic attractive force is weakened, and thus reproduction sound pressure is lowered.
Further, a process of manufacturing the diaphragm for an electrostatic speaker according to the related art is complicated. In addition, in the process of adhering the diaphragm in a tensile state to the frame at a temperature of about 100° C. or more according to the related art, it is difficult to maintain the tensile state of the diaphragm since a polyester film, which is the main ingredient of the diaphragm, has low thermal resistance, and it requires a lot of time to adhere the diaphragm to the frame due to a very stable chemical structure.
In the high resistance conduction coating method using a polymer film, such as a polyester film, it is difficult to perform uniform coating, resulting in charge being concentrated on a portion of the diaphragm. As a result, discharge occurs easily, and arc accompanied with the discharge melt the polymer film, which results in a short life span of the diaphragm. In order to solve these problems, a method of arranging the diaphragm and the fixed electrode at a sufficient distance therebetween not to generate discharge has been proposed. However, this method is not a solution to these problems since the sensitivity of the speaker is lowered.
In addition, the speaker needs to have a characteristic of radially and uniformly spreading sound. However, since the movement of the diaphragm of the electrostatic speaker is done as the planar vibration, it is difficult to naturally spread sound due to the directivity of a high-pitched sound.
In order to solve these problems, in the electrostatic speaker according to the related art, a panel is formed in a semi-cylindrical shape having a length corresponding to the height of a human being, or when the length of the panel is smaller than the height of the human being, the panel is inclined backward so as to be the same height of the listener's ear where the sound is spread. However, since the diaphragm is formed in a semi-cylindrical shape, it is difficult to perfectly realize a radial and uniform sound spread. In addition, a method of additionally providing an electrical delay circuit to operate the diaphragm like a spherical surface has been proposed. However, this method has problems in that the size of a speaker increases, the structure of the speaker becomes complicated, and the amount of data to be processed increases.